Field of the Invention
The present invention relates to a method for preparing ultrathin silver nanowires, and for preparing a transparent, conductive electrode film based on the ultrathin nanowires. More particularly, the present invention relates to a method for preparing silver nanowires, having a diameter of 30 nm or less with a narrow diameter distribution and an aspect ratio of 300 or higher wherein, by such methods, the wires are restrained from growing beyond a certain thickness and are grown in a controlled way so as to provide a wire with an improved aspect ratio.
Description of the Related Art
Ambitious development has been ongoing in the electronic display device industry, with active research focused on cost reduction in thin film preparation and the flexibility, slimness and functionality of such thin films.
To gain a competitive edge, various industries concerning organic solar cells, and organic semiconductors, as well as flat panel displays (FPD) such as liquid crystal displays (LCD), plasma display panels (PDP) and electroluminescent displays, have developed functional materials that are thinner and more flexible than conventional materials, and which are combined to perform various complex functions. Therefore, there is a need needed for development of simpler techniques for preparing such functional materials.
Technologies for functional thin films are particularly widely applied to substrate electrode materials and organic conductors. Recently, film technology for flexible displays as well as organic semiconductors has attracted keen interest.
On the whole, transparent electrode materials refer to materials that are used as transparent electrodes in devices such as flat panel displays and solar cells. For use in such devices, transparent electrodes should have a visible light transmittance of 80% or higher, and be of high electrical conductivity, with a surface resistance of 100 Ω/□ (ohm/square) or less.
Currently, transparent electrodes are prepared mostly from metal oxides via sputtering. In recent years, conductive polymers or carbon nanotubes (CNTs) have been reported as materials of transparent electrodes.
However, these materials are observed to be lower in conductivity, higher in light absorbance, and poorer in chemical and thermal stability than the metal oxide indium tin oxide (ITO). To develop an alternative to ITO, active research has recently been directed toward transparent conductors composed of a random network of silver nanowires.
Silver (Ag) is known to have the highest electrical and thermal conductivity of all metals. When formed at the nano-scale, silver also exhibits excellent optical properties, such as high transmittance of visible light.
For use in the field of transparent electrode materials, silver nanowires should be thin with a high aspect ratio and small size deviation.
In regard to the synthesis of silver nanowires, a method for preparing silver nanowires using a metal catalyst is found in Korean Patent Application Unexamined Publication No. 10-2011-0072762 in which a precursor solution containing an Ag salt, an aqueous polymer, a metal halide with a standard reduction potential of −0.1 to 0.9 V as a metal catalyst, and a reducing solvent is heated to prepare silver (Ag) nanowires.
However, this method cannot restrain the growth of silver nanowires in a thickness direction, which leads to the impossibility of increasing the aspect ratio of the silver nanowires to a certain level. Thus, the conventional technique is improper for preparing silver nanowires to be used as a transparent electrode having a small diameter and excellent aspect ratio.
In addition, techniques relevant to silver nanowires are disclosed in U.S. patent application Ser. Nos. 11/504,822, and 11/871,721, which describes the preparation of silver nanowires using polyol methods.
Also, the prior art describes the synthesis of one-dimensional silver wires in a solution phase using a reducing solvent containing a silver precursor and ethylene glycol, and a capping agent containing polyvinylpyrrolidone (PVP).
Korean Patent No. 10-1089299 introduces the use of an ionic solution of imidazole halide in the polyol synthesis of silver nanowires with a diameter of 80 to 100 nm.
When synthesized using such conventional techniques, the diameter of silver nanowires becomes thick as they grow. Silver nanowires with large diameters are prone to light scattering, thus decreasing their light transmittance. A film formed with thick nanowires thus has poor light transmittance and high haze. Hence, many problems arise when the silver nanowires synthesized by the conventional methods are applied to transparent electrode films.
In conventional preparation processes, as described, silver nanowires tend to become shorter as they become thinner. There is therefore a need for a method of preparing silver nanowires having a high aspect ratio.
The following documents may be relevant and are incorporated by reference:
(Patent Document 001) Korea Patent Application Unexamined Publication No.: 10-2011-0072762 (issued on Feb. 2, 2012)
(Patent Document 002) Korean Patent No.: 10-1089299 (issued on May 27, 2010)
(Patent Document 003) U.S. patent application Ser. No. 11/871,721 (issued on Sep. 13, 2011)
(Patent Document 004) U.S. patent application Ser. No. 11/504,822 (issued on Dec. 31, 2013)